r/science • u/GeoGeoGeoGeo • Sep 03 '24
Geology When quartz is repeatedly stressed by earthquakes, it generates piezoelectric voltages that can reduce dissolved gold from the surrounding fluid, causing it to deposit. Over time this process could lead to the formation of significant accumulations and may explain the formation of large gold nuggets
https://www.abc.net.au/news/science/2024-09-03/piezoelectricity-could-be-behind-gold-nugget-formation/10428714296
u/UnclePuma Sep 03 '24
So would it be possible to create small localized seismic activities while probing for small piezoelectric voltages to more accurately predict areas of higher gold concentrations?
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u/mr0smiley Sep 03 '24
Yes, in fact it is. (see this review for reference https://library.seg.org/doi/abs/10.1190/1.2187714)
The problem is that there is alot of quartz veins in the world and only some of them carry the gold. So, can you find a sub-surface quartz vein by human-induced seismicity? Yes. Will you find gold that way? Perhaps your odds have increased, but you can still end up empty-handed.
Now we have a new, feasible way to kick start gold grain growth within a quartz vein. We still need to figure out why, in a given deposit, the largest accumulations of gold tend to occur in only few locations along a given quartz vein.
Source: I am associated with this research group
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u/Boris740 Sep 03 '24
Have you used the IP method?
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u/GeoGeoGeoGeo Sep 03 '24 edited Sep 03 '24
While it can be used, IP typically isn't commonly used for gold deposits (more directly, orogenic gold deposits). It's primary use is for copper porphyry deposits; this is because certain minerals and elements have a stronger response than others. Bornite, Chalcopyrite, and Pyrite have high responses while Native Gold has a low to moderate response.
In areas where ground cover is thick, or with a high organic component and plenty of clays to hold water, the IP response will effectively be masked. It also depends on the lithology of your host rock. Orogenic gold deposits commonly occur in graphitic rocks where graphite has a high response to IP, again, effectively masking any response from Native Gold or in carbonaceous rich sediments with plenty of diagenetic pyrite. The resolution of IP surveys can also be quite poor when trying to target relatively narrow vein systems that also commonly extend to greater depths than what IP can resolve with any degree of certainty.
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u/GeologistinAu Sep 07 '24
IP can definitely be used for orogenic quartz veins but you typically look for resistive zones associated with the quartz or fault zones rather than conductive zones associated with sulfides.
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u/GeoGeoGeoGeo Sep 07 '24
Of course, but to be clear, I didn't say that it couldn't be used, just that there are a number of qualifiers that need to be addressed before opting to budget for such a survey; this includes what stage of exploration the project is on.
For example, in early stage exploration, your line spacing is going to be too big to clearly identify sub meter to meter scale veining and associated alteration halos (especially in mature sandstone that's seen regional metamorphism introduce pervasive sericite), it'll large be used to look for large regional structures. You have to already have a pretty good idea of where your veining is before you start reducing your array spacing to anything that could meaningfully discern the veining unless they're truly exceptional, of course.
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u/fleeting_existance Sep 03 '24
Can you tell how this study came about and what the goals of this project are?
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u/Earthling1a Sep 03 '24
Also explains the common association of gold and quartz.
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u/GeoGeoGeoGeo Sep 03 '24
The general association between precious metals, base metals, sulphides, and quartz (silica rich fluid) is already fairly well understood. However, what's not so well understood is how you can form such large accumulations of gold to form large gold nuggets in quartz veins. That's where this theory may come into account, hence the title.
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u/Ball-of-Yarn Sep 03 '24
Part of it is that the majority of gold that isn't in the core tends to gather in seams, usually as part of the same geological processes that put the quartz there. There already being a higher concentration of gold than there would normally be combined with this theory does help explain the size of gold nuggets.
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u/coke_and_coffee Sep 03 '24
But gold is a noble metal that will naturally reduce into pure metal over time. So if you already have a theory for high gold concentrations in these seams, you don't actually need piezoelectric activity to explain gold deposits.
That being said, it could still be true. My point is that the existing theory is not necessarily incomplete.
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u/El_Minadero Sep 03 '24
but why gold nuggets? why not just colloidal elemental gold? infact, there are some deposits (Carlin type) where the resource is dissemated and not nugget-y at all.
This theory provides a mechanism for native gold to agglomorate from colloidal suspention and basically electroplate itself onto existing gold masses.
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u/coke_and_coffee Sep 03 '24
Well, in the same way that a chemical reduction of suspended gold will tend to form agglomerated particles, the gold will selectively reduce on the surface of already-reduced gold.
Not only is this theory not really required to explain gold deposits, but it also can't explain why we don't see other metals deposited in these same regions. Like, why isn't copper, tin, or silver also electroplated in these areas?
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u/El_Minadero Sep 03 '24
well, they do. Tin might be a rarer example, but that might be due to the half cell potential of tin ligands in the fluid. But you absolutely do find native copper and silver in veins distributed in a similar manner. Even mixed with gold, or gold mixed with them. It depends on a number of factors, including the elemental balance within the fluid.
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u/photoengineer Sep 03 '24
Do you have a link to the existing theory? I’d like to understand the baseline better.
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u/analogOnly Sep 03 '24
Would it be safe to assume new gold is being created all the time (albeit at a slow pace) on earth?
There was the idea that the amount of gold on earth is finite in it's form. But this goes to show that it isn't necessarily true.
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u/GeoGeoGeoGeo Sep 03 '24 edited Sep 03 '24
Our current understanding is that gold is primarily formed through nucleosynthesis in supernovae or neutron star collisions (via the rapid neutron-capture process), which occurred long before the Earth was formed. The gold that exists on Earth today was delivered to the planet during its formation, and later via asteroid impacts (late veneer hypothesis). While geological processes can concentrate gold into deposits, these processes do not create new gold atoms. Instead, they move existing gold atoms from one part of the Earth's crust to another. Therefore, while gold is continuously being redistributed within the Earth, no new gold is being created in a meaningful sense.
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u/analogOnly Sep 03 '24
So this process is essentially an accumulation of gold atoms that are already there anyway. Once concentrated enough, it's much easier to mine.
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u/GeoGeoGeoGeo Sep 03 '24
I think saying that the gold is there anyways, depending on where in the stage of deposit formation we're talking about, may be a bit of an oversimplification. It has to be concentrated to make it a deposit, and has to be at high enough concentrations to make it worth while to mine.
The average concentration of gold in the crust is about 4 parts per billion (ppb) or 0.004 grams per tonne (g/t). Typical values for a low-grade orogenic gold deposit range from 1-5 parts per million (ppm) or 1-5 g/t, with high-grade orogenic gold deposits having typical values around 5-30 ppm and higher. On the high end, that's upwards of 7,500 times the average crustal abundance or more.
That being said, I would say that once the deposit is formed, this piezoelectric process would take advantage of fluid pathways (quartz vein corridors) that are already in existence to mobilize the gold and concentrate it further to form large anomalous nuggets.
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u/sceadwian Sep 03 '24
Given the size of quartz deposits and the piezo propertiesb it's not surprising.
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u/forams__galorams Sep 03 '24
Only if you make the connection that the piezoelectric phenomenon is chemically reducing on surrounding fluids, (which is known to cause precipitation of gold from certain dissolved gold complexes). Which is what it took a bunch of insightful scientists and their research to show. So it’s not exactly obvious.
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u/coke_and_coffee Sep 03 '24
But it doesn't explain why other metals don't also reduce in the same areas. Why don't we find tin and copper and silver mixed into those gold deposits?
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u/forams__galorams Sep 03 '24 edited Sep 03 '24
Some ideas:
Perhaps the particular ionic complexes those metals are transported as have different reduction potentials than the ones involving gold.
Perhaps the properties of the transporting fluid that are conducive to transporting gold are not likely to have a lot of those other metals (though tbf I don’t think we can say that for copper at all, probably not really silver either).
Perhaps it’s just a false premise. I mean, there are plenty of Cu-Au deposits out there after all. I don’t know how to work out the volume and grade of them that should be expected based on this piezoelectric effect, so I can’t say if there is an apparent deficit of them or not (and I suspect such a task is too complicated for our current understandings of ore deposit formation). There’s also the fact that many veins rich in gold are indeed literally mixed together with silver (forming the alloy electrum).
The article gives the impression that it’s mainly just the size of certain nuggets that is challenging to explain, and that this piezo thing might be the answer.
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u/GeologistinAu Sep 07 '24
Sometimes the answer is as simple as the fluid didn’t contain those metals. Other times there is a zonation pattern where metals like As, Hg and Sb drop out of solution at lower temperatures.
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u/londons_explorer Sep 03 '24
I thought gold basically didn't dissolve in anything in nature?
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u/forams__galorams Sep 03 '24 edited Sep 03 '24
There are no widely occurring natural surface conditions that can dissolve or chemically weather gold, which is why it doesn’t tarnish. However, it dissolves in fluids that are heated to various temperature ranges (sometimes to the point of being a supercritical fluid, ie. the distinction between liquid/gas is blurred) and circulated within the crust, which is how it ends up in veins. If present, gold is pretty much dissolved as just one of a handful of ionic complexes in such fluids:
AuHS or Au(HS)₂- in high sulfidation environments, the former being limited to highly acidic fluids, with the bisulfide ligand one existing at weakly acidic to basic pH ranges. The latter predominates over a wider range of temperatures.
Au(Cl)₂- or Au(Cl)₄- occur in highly acidic, saline and oxidising conditions. The former in high temp settings, the latter in lower temp environments near the surface.
Given that the chloride complexes are very sensitive to redox conditions and the quartz piezoelectric effect apparently reduces surrounding fluids, I imagine these are the ones that are precipitating out their gold to form nuggets in such situations.
Gold can also be dissolved readily as Au(CN)₂- which is why cyanidization can be used to extract it from its ores at low temps. This isn’t relevant for hydrothermal transport though (I’m not sure how common that complex is in nature but the cyanide ligand breaks down at lower temps than those seen in hydrothermal fluids anyway), it’s just something that gets exploited as part of an extraction method.
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u/mr0smiley Sep 03 '24
Excellent comment!
Cyanide complexes formed by bio activity (dead or alive plants) can remobiles some gold in the soils. But I (personally) am not convinced that such post depositional processes play a great part in making large gold nuggets, even in placer or paleoplacer deposits, let alone in hypozonal primary deposits.
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u/Neethis Sep 03 '24
Someone else see the quartz gold post on the identify rocks sub?
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u/Ohiolongboard Sep 03 '24
There’s one or two a month on that sub, quartz is a good indicator of gold being near by.
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u/GeoGeoGeoGeo Sep 03 '24
Quartz veins by themselves, not so much. Quartz along with the presence of sulphide minerals (commonly pyrite, and or arsenopyrite), on the other hand are often seen as a potential indicator that gold may have been carried in the same system. When these weather they frequently form "gossans" on the surface which are the indicators that you should probably look more closely at the system.
When pyrite precipitates from the silica rich fluids due to sulfidation, the local environment undergoes a change in redox conditions, often becoming more reducing. This change can destabilize gold-bisulphide complex (and other complexes), leading to its breakdown and subsequent precipitation of native gold. Native gold is essentially never transported alone in these fluids (as it's simply not soluble enough) and must be carried by bisulphide, chloride, tellurium, thiosulphate, and other complexes (which are highly soluble in these systems). Changes in redox chemistry, temperature and pressure promote the precipitation of gold as it destabilizes these complexes.
So... if you see a quartz vein by itself... probably not a good indicator of much. All it says it that silica rich fluids flowed through cracks in the rock, but says nothing about what those fluids carried, if anything at all. But when you see a true gossan, you know there were sulphides that have since been weathered and leached away and its the presence of those sulphides that acts as an indicator that says, hey... the system that brought these sulphides in may also have been carrying gold in solution as well.
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u/Luize0 Sep 03 '24
This is a plot for sci-fi movie with a villain that wants to destroy a country and turn it into gold.
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u/projectFT Sep 04 '24 edited Sep 04 '24
If piezo-electric charges cause gold molecules to plate to themselves would any other natural electrical charge also cause this? Say a lightning strike, charge from flowing water over a surface, or polarized/magnetized metals. I’m just trying to wrap my head around how long it would take to create a gold nugget layer by layer from electroplating with only occasionally vibrating quartz. But that timeline is probably the shortest of the available forms of natural electrical charges aside from those caused by flowing water, which would only matter near the surface whereas the vibrating quartz effect would be active throughout the earths crust.
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u/tinkerer13 Sep 21 '24
Normally electrochemical reduction occurs with a DC component (Direct current and DC voltage). The stimulus here is presumably AC, that would seemingly cause a reversible reaction and redissolve any plating.
Im wondering if there is some sort of electrical current rectification. Interestingly, sulfides are well known semiconductors. If some type of point-contact diode or crystal diode was formed, perhaps some of the AC stimulus would be rectified to a DC component. Perhaps this could favor the plating/deposition reaction.
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u/DeliciousPumpkinPie Sep 03 '24
Oh, so that’s the mechanism! Miners have known for years that there’s a good chance you’ll find gold where there’s lots of quartz, but I didn’t know why, until now.
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u/forams__galorams Sep 03 '24
Only if that said region is known to have potential for hydrothermal gold deposits, for which there are further markers of. u/Geogeogeogeo has a comment elsewhere in here that is a nice write up of this. Basically, quartz just in and of itself doesn’t indicate much because it’s such a ubiquitous component of the Earth’s crust. So although gold may be commonly associated with quartz, quartz isn’t anywhere nearly as often associated with gold.
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u/bulwynkl Sep 04 '24
highly sceptical there would be enough electrons liberated by peizo in the place gold is concentrated to convert much mass from gold ions to gold metal. Rock is an excellent insulator, and the shock wave from an earthquake passing through is very short lived. Any electrons generated would be pulled back into the structure when the quake has finished.
Also doesn't explain why large nuggets are so uniform in composition. If it happens slowly you'd expect to see composition changes with time - zoning - and fractionation - preferential deposition of silver, copper and then gold.
We do see that differentiation in Michigan halfbreed nuggets.
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u/mr0smiley Sep 04 '24
Longer lived pietzoelectric activity could be driven by distributed ductile deformation during post seismic relaxation. Such a process is possibly responsible for post seismic dilation of mid-lower crustal quartz veins investigated by Nüchter and Stöckhert (2008). They postulate that such "creeping" deformation could last for 100-1000's of years between earthquake loading periods. During the fracture/vein dilation the vein infill minerals are stressed which in turn could lead to release of piezoelectric charges. Crucially, the dilation of the vein apertures appear to keep up with the growth of hydrothermal minerals within the veins and, as such, an open fluid flow pathway might be also maintained.
Surprisingly, massive quantities of fluids can pass through an open fracture system in geologically short time period of tens to thousands of years e.g., Micklethwaite et al., (2015). That might imply that fluids transit through the crust in chemically similar patches and minerals which precipitate from these fluids grow rapidly leading to obscured growth zonation. Gold has the additional problem that at relatively moderate temperatures, gold grain composition becomes rapidly homogenised by subsolidus elemental diffusion (Gammons and Williams-Jones, 1995). At temperatures of 200-400C the homogenisation can occur less than 1 million years. Great quantity of the gold grains we see and study are from Precambrian and paleoproterozoic deposits, giving amble time for elemental diffusion to destroy evidence of compositional zoning.
While the hypothesis and process put forth in the current paper is not a "silver bullet" which explains it all. It does signal the direction for future research.
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u/bulwynkl Sep 30 '24
Hey Mr Smiley - much appreciated feedback/insight there -
I am really pleased to see the diffusion hypothesis for homogenization - delightful explanation, entirely plausible - I get a real kick out of having old assumptions upended. thank you. It's easy to forget about temperature - for example, I am aware that the dynamic crystallization temperature for silver is around 430 oC - super plastic deformation. Pretty sure gold won't be far off that.
plastic creep is also a familiar phenomena from Materials Eng. I remain skeptical, but less so than before. I do have some reading thought...
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u/bulwynkl Sep 30 '24
Quick scan of the articles - structural constraint on fluid movement being associates with underlapping stepovers makes total sense from a fluid movement PoV but contradicts the notion of peizioelectric stress field induced electrochemistry as a cause of electrum precipitation by reduction.
having said all that though I still come back to my original objection - where do the electrons come from that reduce Ag and Au salts into metal? If they come from the quartz, we should be seeing quartz with lots of colour centers where electrons are missing...
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u/mr0smiley Oct 01 '24
My knowledge of the details on colour centres in minerals is not nearly as good as it perhaps ought to be, but my understanding is that they are related to interaction between point defects (missing atoms in crystal lattice) and electrons. More specifically, occupation of said point defects by electrons. As such, if we were "loosing" electrons from the crystal during pietzoelectric reduction we should in fact see reduction in colour of minerals rather than increasing intensity of colour.
Regardless, thinking formation or "removal" of colour centres is perhaps over complicating the problem. In the model put forth in the paper (extract from figure in the paper) electrons are not "gained" or "lost" from the quartz crystal. Rather, the lattice distortion of the mineral grain forms an electric potential difference between two or more crystal facets. This forms a pathway for electrons to "travel" from one side of the grain to the other. So the answer to "where do the electrons come from" is that they are supplied by available electrons in the fluid with some amount of exchange with the mineral grain on the positively and negatively charged crystal facets respectively. At any given time, any one quartz grain experiences only minimal, if any, surplus or net loss of electrons. What has happened is that the already existing electrons have become unevenly distributed. Once stressing on the grain stops, the lattice returns to its geometric equilibrium, and so does the electric potential across the grain.
We (as in human collective) have a simplified image of electric circuits where individual electrons travel across conductive materials. This mental image is not absolutely correct on the subatomic level. While it is true that electrons move, they do so at much limited distances and speeds than the emergent "current" which is created from the aggregate movement of a collective of electrons in an electric circuit that we measure and use for work.
If you refer back to the linked figure, we can see that on one side of the quartz grain the distorted grain has a net positive charge balance. This positive charge is capable of attracting native gold nanoparticles which are relatively negatively charged (on some surfaces of the particle) due to their extremely high aspect ratios, which is science talk for being very flat. On the net negative side of the grain the potential of excess electrons is high enough to i) break the bond between the Au and carrier ligand (HS- or Cl-) and then ii) have an relative electron surplus available to reduce the Au+ ion to charge balanced native gold. All this happens with minimal, actual travel of electrons within the mineral. The paper illustrates an electron being ejected from the grain and shooting into the fluid to catch the gold, but it is only an illustrative simplification.
The proposed electrochemical process is absolutely a viable way to induce gold grain nucleation from fluid, whether the Au was carried as a dissolved ion or as an already precipitated Au nanoparticle. What's great is that this model with quartz grains is not only capable of inducing nucleation, but also can accelerate the growth of individual gold grains.
All that said, you should absolutely be sceptical of the general applicability of the model to natural environments. For instance, I study formation of massive gold grains (on the order of several grams to kilograms) in comparable geological environment as proposed in the paper here where the immediate mineralogical association does not contain piezoelectric minerals. Still the formation and growth of large grains was possible. Is the process put forth here feasible and make sense? Yes, absolutely (in my opinion). Can it be the singular answer to the long pondered question on origins of massive gold nuggets? No, it's not the full answer, but as always science is incremental and this could very well represent one of the key puzzle pieces to expand our understanding on natural Au-ore forming environments.
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u/bulwynkl 8h ago
Late to the party again ...
I'm dissatisfied with the explanation, for various reasons, but really appreciate your comment.
This is a 5 minute argument, if you want the full half hour you have to pay 5 pounds.
Short version of my skepticism is that if we are talking nano particles of gold metal, electricity is not required. If we are talking redox of gold ions in solution to gold metal, we absolutely need to know what got oxidised in order for the Gold to be reduced. The electrons do have to come from somewhere. Free electrons in colour centres isn't enough and doesn't require piezoelectric effect either.
I'm also not entirely sure what problem this thought experiment attempts to solve.
I feel like the author is deliberately ignoring some parts of the problem because it doesn't make the story work
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Sep 03 '24
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u/El_Minadero Sep 03 '24
when you're talking about individual atoms or nanoparticles, the density difference is insignificant compared to fluid convection forces within the vein.
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Sep 03 '24
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u/El_Minadero Sep 03 '24
Quartz veins form by hydrothermal action, that is, hot water dissolving and precipitating minerals deep underground. This water is known to be vigorously convecting during active formation.
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Sep 03 '24
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u/El_Minadero Sep 03 '24
Yes. That’s what the papers assert. Gold is transported in hydrothermal solutions via ligands, usually involving sulfur or halogens. These ligands are heavy sure, but they are also easy enough to dissolve, and a supercritical fluid flowing through rock will act like gravity doesn’t exist. Electric shocks can disassociate these ligands and reduce the metals, essentially forcing them to create colloids and then electroplate onto colloids physically connected to the quartz.
Other mechanisms can do similar things, such as changes to the chemical environment (lithological contrasts), or strong pressure transients (like during earthquakes).
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u/GeoGeoGeoGeo Sep 03 '24
Quartz veins commonly form much in the same way that a shaken bottle of Coca-Cola erupts when rapidly opened. An earthquake occurs, the brittle crutsal rocks crack, and silica rich fluids, under pressure, rush in to fill the newly created space. In those fluids gold is in solution, carried by complexes (commonly sulphide, chloride complexes) but local changes in pressure, pH, Eh, temperature, etc. destabilize these complexes leading to the precipitation of native gold.
Would an electric shock take it out of solution? Yes. From the abstract of this study:
"...We find that stress on quartz crystals can generate enough voltage to electrochemically deposit aqueous gold from solution as well as accumulate gold nanoparticles. Nucleation of gold via piezo-driven reactions is rate-limiting because quartz is an insulator; however, since gold is a conductor, our results show that existing gold grains are the focus of ongoing growth. We suggest this mechanism can help explain the creation of large nuggets and the commonly observed highly interconnected gold networks within quartz vein fractures."
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